This invention has been made with the support of National Science Foundation Grant No. CHE-8908136. The U.S. Government has certain rights in the invention.
Triaryl isocyanurates of general formula 1: ##STR1## wherein C.sub.6 H.sub.4 is 1,4-phenylene, 1,2-phenylene or 1,3-phenylene and R is, for example, 2-, 3- or 4-halo, H, methyl or methoxy, are useful as activators for the continuous anionic polymerization and post-polymerization of .epsilon.-caprolactam to nylon-6. These activators yield a final product having a low content of unreacted monomer and a highly stable melt viscosity. See, for example, Z. Bukac et al., Czech. CS 227,247 (Chem. Abstr., 105, 173224r (1986)); Z. Bukac et al., Chem Prum., 35, 361 (1985) (Chem. Abstr., 103, 123978c (1984)); J. Horsky et al., Int. Polym. Sci. Tech., 9, 65 (1982). Recently, the superior thermal and hydrolysis stability of triphenyl isocyanurate-based foams and plastics have generated considerable interest in the development of methods to produce trimers of general formula 1. See, H. Ulrich, J. Cellular Plastics, 17, 31 (Jan./Feb. 1981), P.I. Kordomenas et al., Macromolecules, 14, 1434 (1981) and D.K. Hoffman, J. Cellular Plastics, 20, 129 (1984).
Since impurities in the activators of formula 1 lower the quality of nylon-6, attempts have been made to develop purification methods for these trimers. However, due to the complexity of the processes which have been used, only relatively low yields of pure products have been obtained. See, Z. Bukac et al., cited above.
Several catalytic methods to prepare triaryl isocyanurates have been reported. For example, Y. Taguchi et al., Bull. Chem. Soc. Japan, 63, 3486 (1990) reported the trimerization of phenyl isocyanate in the presence of amine catalysts in 22-100% yield using high pressures. S. Kato et al., J. Orqanometallic Chem., 51, 167 (1923) trimerized phenyl isocyanate to yield 1 (R.dbd.H) in 82% yield, using [.alpha.-(trimethylstannyl)phenacyl]triphenylphosphonium ylide. E. Martilli et al., J. Molec. Catal., 22, 89 (1983) reported the use of (.eta..sup.5 -C.sub.5 H.sub.4 Me)Mn(CO).sub.3 and photolysis to catalyze the same reaction in 80% yield. J. Mizuya et al., J. Polymer Sci.: Part A: Polymer Chem., 29, 1545 (1991) accomplished the same reaction in relatively low yields (72-80%) using large amounts of alkoxyalkenes as catalysts. However, the more electrophillic isocyanate, 4-methylphenyl isocyanate, did not cyclotrimerize under these conditions. K. Ashide, EPA 169,708 (Chem. Abstr., 107, 134825j (1987)) trimerized phenylisocyanate to 1 (R.dbd.H) in only 63% by using 10% silicates as the catalyst.
Therefore, a continuous need exists for methods to prepare triarylisocyanurates in high yields, which require little or no purification of the final product. A further need exists for methods to prepare triisocyanurates under mild reaction conditions using non-toxic, non-metallic catalysts.